1. Field of the Invention
The present invention relates to a process for preparing at least one partial oxidation and/or ammoxidation product of a hydrocarbon by subjecting at least one saturated hydrocarbon H to a heterogeneously catalyzed dehydrogenation in the gas phase to form a product gas mixture A which comprises at least one partially dehydrogenated hydrocarbon H, leaving constituents present in the product gas mixture A, other than the saturated hydrocarbon H and other than the partially dehydrogenated hydrocarbon H therein, or partly or fully removing them to obtain a product gas mixture A′, and subjecting product gas mixture A and/or product gas mixture A′, as a constituent of a gas mixture B, to at least one heterogeneously catalyzed partial oxidation and/or ammoxidation of the at least one partially dehydrogenated hydrocarbon H present in the product gas mixture A and/or product gas mixture A′.
2. Description of the Background
In this document, a heterogeneously catalyzed dehydrogenation refers to a dehydrogenation which proceeds endothermically and in which the primary by-product formed is hydrogen. It is carried out over solid catalysts which reduce the activation energy required for the thermal cleavage of a C—H bond. The heterogeneously catalyzed dehydrogenation differs from a heterogeneously catalyzed oxydehydrogenation in that the latter is forced by oxygen present and water is formed as the primary by-product. In addition, a heterogeneously catalyzed oxydehydrogenation proceeds exothermically.
In this context, a complete oxidation of a hydrocarbon means that all of the carbon present in this hydrocarbon is converted to oxides of carbon (CO, CO2).
Reactions with oxygen which deviate from this are partial oxidations and, in the presence of ammonia, partial ammoxidations.
The process described at the outset is known (cf., for example, DE-A 10219686, DE-A 10246119, DE-A 10245585, DE-A 10219685, EP-A 731077, DE-A 3313573, DE-A 10131297, DE-A 10211275, EP-A 117146, GB-A 2118939, U.S. Pat. No. 4,532,365, U.S. Pat. No. 3,161,670 and EP-A 193310) and is employed, inter alia, for preparing acrolein, acrylic acid and/or acrylonitrile from propane, methacrolein, methacrylic acid and/or methacrylonitrile from isobutane. The partial ammoxidation differs from the partial oxidation essentially by the presence of ammonia in the reaction mixture. Suitable choice of the ratio of NH3 and O2 can allow partial oxidation and partial ammoxidation also to be carried out in parallel, i.e. simultaneously. Addition of inert diluent gases keeps the reaction mixture of the partial oxidation and/or ammoxidation outside the explosion range.
In this process (in particular in accordance with the teaching of DE-A 3313573, EP-A 117146, GB-A 2118939, U.S. Pat. No. 4,532,365 and U.S. Pat. No. 3,161,670), either product gas mixture A as such and/or product gas mixture A′, as a constituent of a gas mixture B, can be subjected to at least one heterogeneously catalyzed partial oxidation and/or ammoxidation of the at least one partially dehydrogenated hydrocarbon H present in the product gas mixture A and/or product gas mixture A′.
In the simplest case, the product gas mixture A can be converted to the product gas mixture A′ by partly or fully removing any steam present in the product gas mixture A. This can be effected, for example, by cooling the product gas mixture and partly or fully condensing out any steam present in it.
It will be appreciated that other constituents present in the product gas mixture A can also be removed to obtain a product gas mixture A′. For example, the product gas mixture A can be passed through a membrane, generally configured as a tube, which is permeable only for hydrogen present in the product gas mixture A, thus partly or fully removing hydrogen present in the product gas mixture A. CO2 present in the product gas mixture A can be removed by conducting the product gas mixture A through an aqueous alkali solution. An alternative removal method is absorption/desorption (stripping) according to DE-A 10245585.
However, a disadvantage of the prior art processes is that neither the product gas mixture A nor the product gas mixture A′ nor the gas mixture B, before the at least one heterogeneously catalyzed partial oxidation and/or ammoxidation, is subjected to a mechanical separating operation by which solid particles present in these gas mixtures can be removed from these gas mixtures. When an absorptive separating process is employed, the absorbent in some cases becomes saturated over time with solid particles. For example, in the course of stripping and/or desorbing, they can be entrained.
As a long-term in-house experiment found, surprisingly, this is disadvantageous in that, in the processes described at the outset, very fine particles of the solid catalyst this is disadvantages in that used for the heterogeneously catalyzed dehydrogenation can be conveyed into the subsequent heterogeneously catalyzed partial oxidation and/or ammoxidation, where they in some cases settle in the fixed catalyst bed used there.
The latter is disadvantageous in that a heterogeneously catalyzed partial oxidation and/or ammoxidation, relative to the reaction stoichiometry, is normally carried out in the presence of an excess of oxygen.
In the presence of oxygen, catalysts suitable for a heterogeneously catalyzed dehydrogenation normally also catalyze complete combustion of hydrocarbons to CO2 and H2O (cf., for example, U.S. Pat. No. 4,788,371) and the hydrogen-oxygen gas reaction of H2 with O2 to give H2O. Both of these are disadvantageous in that they lead either in an undesired manner to undesired reactant consumption in the heterogeneously catalyzed partial oxidation and/or ammoxidation (which at the same times means, undesirably, additional heat formation) or harbors risks which can only be estimated with difficulty.